The levels of some trace metals have been studied in Southampton Water. Particulate iron, though variable, is generally high and the concentration appears to be correlated with the amount of particulate matter. Zinc is approximately doubled in concentration inside Southampton Water, but much higher levels are occasionally encountered. Copper and nickel are only somewhat higher and total mercury is lower inside Southampton Water than in Solent waters. Zinc and copper are concentrated approximately 30000 times on a dry mass basis by Mercenaria mercenaria. Mercury is concentrated by Mercenaria and other bivalves; the mud which is especially rich in total mercury may represent an important source of mercury for these animals. Primary nutrients, phosphate, nitrate and ammonium, increase in concentration from seaward on proceeding up the estuary. Surface waters appear to be especially rich in nitrate and ammonium. Gross pollution seems unlikely in view of low nitrite concentration and high oxygen values virtually throughout the estuary. The high rate of turnover of organic substrates by heterotrophic organisms may be associated with relatively large amounts of organic matter in Southampton Water. This has to some extent been confirmed by direct surveys of particulate and dissolved organic carbon which also suggest that regional differences exist. Higher levels of organic matter occur near the head of the estuary, near Marchwood, the Docks, and the mouth of the River Itchen. Although thermal changes in Southampton Water appear to have been slight, some increase in winter minimal temperatures and in summer maxima have occurred, especially in the Marchwood area. No evidence exists of deleterious effects on marine organisms. Phytoplankton can apparently endure rises of at least 10 degrees C for short periods without injurious effects. The density of zooplankton has not decreased; possibly certain species (e.g. Elminius modestus) have been favoured by thermal changes. A marked increase in Mercenaria mercenaria population may be associated with the small thermal rise and remarkably high population densities are encountered. Spawning appears to be correlated with summer temperatures exceeding 18 to 19 degrees C.